Magnetic resonance phenomena occurs in a magnetic environment that is controllably homogeneous: uniform in magnitude, direction and stable in time. This environment is known as the polarizing field and imparts to the chaotically directed nuclear spins, a preferred direction in space around which the variously directed nuclear spins precess. Much effort has been directed to the production and control of the polarizing field.
The resonant absorption of energy from an external source occurs through the agency of an RF magnetic field applied to the nuclei under study at an angle (preferably 90° to the polarizing field) through an RF resonator. Substantial effort has been expended to produce and control the homogeneity of this RF magnetic field associated with this resonator as experienced by and with the nuclei of the sample under study. The shape, material and motion of the vessel containing the (liquid) sample has been studied and taken as a subject for further advancement of the homogeneity of the environment of the sample. The present work concerns this latter area of sample shape for further innovation.
The RF resonator has been a fertile ground for development over many decades. The present work is intended for the case where the RF magnetic field associated with the resonator is in the plane normal to the polarizing field. The form of resonator for this arrangement is known as a saddle shaped coil and the present work is limited to operation with such saddle coils. It should be recognized herein that the role of the RF resonator is understood to encompass provision for signals corresponding to either or both excitation and de-excitation of the nuclei under study.
In conventional practice, liquid samples for investigation via NMR are presented in long cylindrical tubes along the axis of the RF saddle coil of the NMR probe. When the RF coil is saddle shaped, the direction of the RF magnetic field (within the volume defined by the coil) is transverse to the long axis of the cylindrical sample vessel. FIG. 1a shows a schematic representation of the coil elements, the sample vessel and RF magnetic field in cross section for a conventional arrangement. This figure also shows the common arrangement of separate coaxially disposed coils. As shown, these coils produce respective RF fields on orthogonal directions in the plane transverse to their common axis.
An improvement to this conventional arrangement appears when the filling factor of the coil (the volume of the sample in respect of the interior volume defined by the coil) is optimized through allowing the inner dimensions of the RF coil to more closely approach the outer dimensions of the sample vessel. Moreover prior art recognized that the RF magnetic flux is substantially homogeneous within the inner confines of the RF coil and would be even more so were the cross section of the RF saddle coil to be deformed from conventional quasi-arc sectors (in cross section) of FIG. 1a, to planar segments and the sample vessel cross section similarly deformed to an elongate cross sectional shape (ellipsoidal or rectangular) in conformity with an elongate (ellipsoidal or rectangular) coil cross section. This geometry is intended to produce a greater degree of homogeneity in the RF magnetic field of the saddle coil and to yield an extended volume space wherein such homogeneity obtains. Such prior art is illustrated in FIGS. 1b and 1c and more description appears in the U.S. Pat. Nos. 7,068,034 and 6,917,201, both assigned to Varian, Inc.
Lossy samples present a case of particular concern. Such samples exhibit a significant electrical conductivity and under the influence of RF electric fields there results RF currents, which contribute noise (for example, from the magnetic fields associated with these currents) and thus degrade the sensitivity of the NMR instrument. The geometrical region of a sample producing the greatest contribution to signal is that region of the highest RF magnetic field amplitude, which may be identified with a region proximate the saddle coil axis. As the sample cross section is increased, more sample may be included, but the influence of the RF E field will be more effective in producing noise in that portion of the space more remote from the central region. The signal to noise figure therefore suffers. For study of lossy samples, it has been common practice to present the sample in a 3 mm sample tube to minimize the effect of RF electric fields by confining the sample to close proximity to the RF coil axis. In such arrangements, sensitivity is inherently compromised by the dearth of sample volume.
The prior art sample vessel of rectangular cross section is difficult to manufacture to the uniform close tolerances necessary for application to high field/high resolution NMR analysis. By way of comparison, conventional cylindrical sample vessels of 3 mm O.D. are commercially offered with a concentricity specification of 0.0005 and a camber specification of 0.00025 over axial lengths of 8 inches (203.2 mm). To approach equivalent tolerances rectangular cross section vessels must be selected from a great number of units at considerable expense. Indeed, the specification of outer dimension and wall thickness for cylindrical tubes is more easily achieved than the specification of two outer dimensions and inner area for the prismatic tube. Accordingly, it is desired to obtain the benefit of the matching a transversely elongate sensitive volume (associated with a saddle coil) to an elongate sample volume in a reliably reproducible and inexpensive manner.
In another prior art sample cell (U.S. Pat. No. 5,552,709, assigned to Varian, Inc.), the same sample for analysis fills a plurality of separate sample holding structures for analysis. The multiple cells are intended to reduce the electrical current paths through lossy sample solutions. The array of closely packed sample vessels is uniform in cross sectional distribution and provides no benefit from alignment of the shape of a macro-sample in respect of the RF magnetic field.